Ceramic investment casting is a specialized form of investment casting that is used to produce high quality metal articles that meet relatively close dimensional tolerances. Investment casting has also been called lost wax, lost pattern, and precision casting. Typically, an investment casting is made by first constructing a thin-walled ceramic mold called an investment casting shell into which a molten metal can be introduced.
Shells are usually constructed by first making a facsimile or pattern from a meltable substrate of the metal object to be made by investment casting. Suitable meltable substrates may include, for example, wax, polystyrene or plastic.
Next, a ceramic shell is formed around the pattern. This may be accomplished by dipping the pattern into a slurry containing a mixture of liquid refractory binders such as colloidal silica or ethyl silicate, plus a refractory flavor seed as quartz, fused silica, zircon, alumina and aluminosilicate and then sieving dry refractory grains onto the freshly dipped pattern. The most commonly used dry refractory grains include quartz, fused silica, zircon, alumina and aluminosilicate.
The steps of dipping the pattern into a refractory slurry and then sieving onto the freshly dipped pattern dry refractory grains may be repeated until the desired thickness of the shell is obtained. However, it is preferable if each coat of slurry and refractory grains is air-dried before subsequent coats are applied.
The shells are built up to a thickness in the range of about 3/16 to about 1/2 of an inch (from about 0.47 to about 1.27 cm). After the final dipping and sieving, the shell is thoroughly air-dried. The shells made by this procedure have been called "stuccoed" shells because of the texture of the shell's surface.
Next, the shell is heated to at least the melting point of the meltable substrate. In this step, the pattern is melted away leaving only the shell and any residual meltable substrate. The shell is then heated to a temperature high enough to vaporize any residual meltable substrate from the shell. Usually before the shell has cooled from this high temperature heating, the shell is filled with molten metal. Various methods have been used to introduce molten metal into shells including gravity, pressure, vacuum and centrifugal methods. When the molten metal in the casting mold has solidified and cooled sufficiently, the casting may be removed from the shell.
The solidification rate of the molten metal in an investment casting mold significantly affects the microstructure, strength and quality of the casting. If the solidification rate is too rapid, the metal may not have enough time to release trapped air and completely fill a mold. If the solidification rate is too slow, the casting may shrink excessively and exhibit a coarse microstructure. These drawbacks, as well as others, may be avoided or minimized by controlling the cooling rate of the molten metal in an investment casting mold.
Currently, some foundries wrap their investment casting molds with high-temperature ceramic fiber blankets in an effort to insulate the casting and decrease the cooling rate of the investment casting within the mold. Wrapping molds with ceramic fiber blankets is a time-consuming process. Moreover, situations where foundry workers may inhale ceramic fibers should be avoided because ceramic fibers are unhealthy to breathe.
On the other hand, to increase the rate of cooling of an investment casting, fans have been used. Each method that is currently used for increasing or decreasing the rate of cooling of an investment casting within the mold has its own particular drawbacks.